AR^OSOMA PYROCHLOA. 187 



of the ambulacral plates. At the ambitus the wiath of the ambiilacral 

 area is 50 mm., and that of the interambulacral 73 mm. There is a narrow 

 bare space running between the ambulacral and the interambulacral areas, 

 and a somewhat wider bare space on the median interambulacral line. On 

 the actinal side there are 34 plates in each half-ambulacrum. These plates 

 carry two principal vertical rows of primary tubercles (PI. 79) ; one row of 

 small tubercles extends from the actinostome above the ambitus in the space 

 separating the single series of pores from the double column, and one row 

 of large tubercles runs along the median ambulacral line. The latter 

 is flanked with an irregular row of smaller primaries extending about half 

 way from the ambitus to the actinal system. The rest of the ambulacral 

 plates carry small secondaries and miliaries irregularly arranged, as on the 

 interambulacral plates. 



The uncalcified membrane between the interambulacral plates has its 

 greatest width about half way between the ambitus and the actinal system. 

 On the abactinal side of the test (PI. 79) the bare space separating the 

 coronal plates is reduced to a narrow line (PI. 80, fig. J^). 



On the actinal side of the test many of the larger primary spines are 

 somewhat flaring at the extremity ; the others are straight and sharp, as are 

 the miliary and secondary spines. On the abactinal side of the test the 

 primary spines are sharp, slender, and straight, very slightly tapering. The 

 miliary and secondary spines are slender and fine ; those of the abactinal 

 part of the test are longer than the others, especially in the interambu- 

 lacral area. 



The greatest diameter of the abactinal system is 26 mm. (PI. 80. fig. 2). 

 The genital plates are comparatively small, very pointed, with a broad plate 

 adjoining the anal system and smaller plates adjoining the genital mem- 

 branes. The larger plates carry from five to seven secondaries, with a few 

 miliaries irregularly arranged; the other plates carry large or small miliaries 

 according to their size. 



The oculars are large, irregularly heptagonal (PI. 80, fig. 2), carrying, 

 next to the anal system, from seven to ten medium-sized secondaries with a 

 few miliaries. The ocular pore is very small. The anal system is covered 

 by three or four unevenly concentric rows of irregularly shaped polygonal 

 plates, carrying small secondaries like those of the genital and ocular plates, 

 and occasionally a few small miliaries, with an inner belt of very small 

 elliptical plates round the anal opening. 



